<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
     .container{
         width:100%;
         display: flex;
     }   
     .left{
         width:100px;
         background:plum;
     }
     .right{
         flex:1;
         background:purple;
     }
</style>
<body>
    <div class="container">
        <div class="left">123</div>
        <div class="right"></div>
    </div>
</body>
</html>